Method for the production of an aluminum diffusion coating for oxidation protection

ABSTRACT

For the production of an aluminum diffusion coating for oxidation protection of metallic components, components are masked in areas not to be coated, and unmasked areas are electrically-plated with aluminum in an aprotic solution at low temperature. Upon removal of the masking material, the components are heat-treated according to a certain high-temperature-time graph such that aluminum elements diffuse into the component alloy and alloy elements of the component diffuse into the aluminum coating. The method involves low manufacturing and material costs and guarantees a defined formation of the aluminum diffusion coating on the component portions to be protected.

This application claims priority to German Patent Application DE102007008011.7 filed Feb. 15, 2007, the entirety of which is incorporated by reference herein.

This invention relates to a method for the production of an aluminum diffusion coating for oxidation protection of metallic components, in particular of aircraft gas-turbine components made of a nickel-base alloy.

Certain engine components, such as hot gas-wetted turbine rotor and stator blades in nickel-base alloy, are, in service, severely attacked by oxidation processes, as a result of which the service-life of the blades is reduced and their replacement or repair is required.

According to a known principle for oxidation protection of such components, a surface-near zone of the base material in the component area to be protected is enriched with aluminum in a deposition and diffusion process, thereby producing an aluminum-oxide coating which protects the respective area from oxidation during further use.

In the known methods for the production of aluminum diffusion coatings, a limited portion of the respective component—the portion which in service is exposed to the hot-gas flow—is inserted in an aluminum-rich gaseous medium in the case of pack aluminising in aluminum powder and in the case of chemical gas-phase deposition (CVD, chemical vapor deposition) in vacuum, with the aluminum being diffused into the metal at temperatures ranging between 900 and 1100° C.

With regard to their specific application, certain portions of the components, such as the highly stressed blade root of a turbine blade, shall not be coated and must, therefore, be covered (masked) during the diffusion process.

The known diffusion processes are disadvantageous in terms of cost in that large quantities of aluminum scrap are, on the one hand, produced in diffusion from aluminum powder in the pack aluminising process and, on the other hand, high apparatus and handling investment and effort is required for diffusion in vacuum according to the CVD process.

Due to the high temperatures involved with the diffusion process, masking of components entails considerable effort and investment. Moreover, if an adhesive is applied for masking, the high temperatures may cause carbon from the adhesive to diffuse into the component, thereby affecting its strength properties.

It is a broad aspect of the present invention to provide a method for the production of an aluminum diffusion coating on metallic components which requires less effort and investment and ensures high component quality.

The basic idea of the present invention is that an aluminum coating of specific thickness is initially produced on the free portions of the masked component(s) using an aprotic solution, i.e. in a water and oxygen-free electrolyte with an aluminum anode and the component(s) as cathode, that the masking is then removed from the component(s) and the component(s) thereafter subjected to heat treatment in protective gas or vacuum according to a certain temperature-time graph during which aluminum diffuses into the component(s) and, reversely, alloy elements of the component(s) diffuse into the aluminum coating, thereby producing an aluminum diffusion coating in the coated portions as protection against oxidation during further use. As opposed to the state of the art, aluminum coating with masking is accomplished at low temperatures in a first process step while, after removal of the masking in a second process step, the unmasked component is heat-treated at elevated temperature in a third process step to produce an aluminum diffusion coating.

Since electro-plating is controlled in thickness and size, the diffusion coating can be produced in a defined manner, in fact, without masking and hence without the need to expose the masking to the high heat-treatment temperatures required for diffusion. The demands on the type and quality of the masking for electro-plating, as it is exposed to only low temperatures during the plating process, are less than those for a masking required for diffusion. Moreover, consequential damage to the components due to the effect of high temperatures on the material of the masking is avoided.

The masking material must only be temperature-resistant up to approx. 400° C.

Prior to electro-plating, the component portions to be coated are cleaned and treated with an activator, preferably nickel chloride.

In accordance with an example of the temperature-time graph at which aluminum diffuses into the component alloy and alloy elements diffuse into the aluminum coating, the components—after being heated to approx. 400 to 700° C. and held up to 2 hours—are heat-treated for approx. one hour at approx. 1100° C. and then for approx. five hours at approx. 1030° C., followed by cooling in undisturbed air.

Further objects and advantages of the present invention will become apparent from the following detailed description of an example of a turbine blade of a gas-turbine engine made of nickel-base material whose airfoil is to be provided with an aluminum-oxide coating as protection against oxidation by the hot working gases and whose blade root, which is held in recesses in the rotor disk, is to remain coating free.

The roots of the turbine blades, which due to their fixation in the rotor disk are subject to high mechanical loads, are covered with adhesive metal tape to protect them against exterior influences during further processing. For masking, the blade roots may also be placed in a box or have adhesive applied to them. Then, the mechanically processed and cleaned turbine blades are treated with an activator, for example NiCl (or a fluoride, preferably potassium aluminum fluoride) to improve the bond of the aluminum coating to be subsequently applied by electro-plating. The turbine blades so prepared are now placed into an aprotic solution (preferably within an oxygen-encapsulated facility), i.e. a water and oxygen-free organic electrolyte with soluble aluminum anodes provided in it, and electrically plated in the area of the non-masked blade airfoils with aluminum, preferably pure, to a coating thickness of 5 to 10 μm. Since the masking is exposed to only very low temperatures (approx. 300° C.) during the electro-plating process, the investment and effort for masking work and masking material, as well as subsequent unmasking, is low, with the relatively low temperatures causing no reaction between masking material and base material, and with consequential damage due to the masking being avoided in the masked material area.

Subsequently, the turbine blades are subject to heat treatment in an oven, actually in protective gas atmosphere or also in vacuum, which, upon heating to approx. 600° C. and holding at this temperature for 1.5 hours, comprises heating to 1100° C. and holding for 1 hour, followed by heating to 1030° C. and holding for five hours, during which diffusion of aluminum from the electrically plated aluminum coating into the nickel-base alloy and of nickel into the aluminum coating occurs. Subsequently, the blades are cooled in undisturbed air in less than 10 minutes. In an oxygen atmosphere, the aluminum at the surface of the Ni—Al diffusion coating so produced on the airfoil forms an aluminum-oxide coating which protects the blades in the engine from further oxidation during future use. The aluminum coating produced by the electro-plating can be subsequently treated in a pickling bath. 

1. A method for the production of an aluminum diffusion coating for oxidation protection of a metallic component, comprising: masking certain portions of the component; applying aluminum to unmasked surfaces of the masked component by electro-plating in a water and oxygen free organic electrolyte; subsequently removing the masking; and then heat treating the unmasked component in at least one of a protective gas atmosphere and a vacuum according to a given high-temperature-time graph, such that in a surface-near zone of the component, the aluminum diffuses into the component and alloy elements of the component diffuse in an opposite direction into the aluminum coating.
 2. The method in accordance with claim 1, wherein the masking includes at least one of adhesives that are heat-resistant below 400° C., synthetics and adhesive metal tapes.
 3. The method in accordance with claim 2, wherein the component portions to be coated are cleaned and treated with an activator prior to electro-plating.
 4. The method in accordance with claim 3, wherein the activator is nickel chloride.
 5. The method in accordance with claim 4, wherein the component for aluminum diffusion heat treating is first heated to approx. 400° to 700° C. and held at this temperature up to two hours, and then further heat-treated for approx. one hour at approx. 1100° C. and then for approx. 5 hours at approx. 1030° C., followed by cooling in undisturbed air.
 6. The method in accordance with claim 5, wherein the unmasked portions of the component are coated in the electrolyte to a layer thickness of 5 to 10 μm.
 7. The method in accordance with claim 6, wherein the aluminum coating produced by electro-plating is subsequently treated in a pickling bath.
 8. The method in accordance with claim 1, wherein the component portions to be coated are cleaned and treated with an activator prior to electro-plating.
 9. The method in accordance with claim 8, wherein the activator is nickel chloride.
 10. The method in accordance with claim 9, wherein the component for aluminum diffusion heat treating is first heated to approx. 400° to 700° C. and held at this temperature up to two hours, and the further heat-treated for approx. one hour at approx. 1100° C. and then for approx. 5 hours at approx. 1030° C., followed by cooling in undisturbed air.
 11. The method in accordance with claim 10, wherein the unmasked portions of the component are coated in the electrolyte to a layer thickness of 5 to 10 μm.
 12. The method in accordance with claim 11, wherein the aluminum coating produced by electro-plating is subsequently treated in a pickling bath.
 13. The method in accordance with claim 1, wherein the component for aluminum diffusion heat treating is first heated to approx. 400° to 700° C. and held at this temperature up to two hours, and the further heat-treated for approx. one hour at approx. 1100° C. and then for approx. 5 hours at approx. 1030° C., followed by cooling in undisturbed air.
 14. The method in accordance with claim 13, wherein the unmasked portions of the component are coated in the electrolyte to a layer thickness of 5 to 10 μm.
 15. The method in accordance with claim 14, wherein the aluminum coating produced by electroplating is subsequently treated in a pickling bath.
 16. The method in accordance with claim 1, wherein the unmasked portions of the component are coated in the electrolyte to a layer thickness of 5 to 10 μm.
 17. The method in accordance with claim 16, wherein the aluminum coating produced by electroplating is subsequently treated in a pickling bath.
 18. The method in accordance with claim 1, wherein the aluminum coating produced by electroplating is subsequently treated in a pickling bath.
 19. The method in accordance with claim 1, wherein the metallic component is an aircraft gas-turbine component made of a nickel-base alloy 